The present invention pertains to an apparatus for classifying powders. In general terms, classification of powders refers to the separation of a feed powder containing particles having a variety of particle sizes into a coarse fraction and a fines fraction in accordance with a selected "cut" size. One known method to evaluate the air classifier's cut size and sharpness is to construct a grade efficiency curve that plots size selectivity (.eta..sub.D) versus particle size (D). The relationship can be calculated by analyzing the particle size distributions of the feed and final product to determine what percentage of a particle size in the feed goes into the coarse fraction. Size selectivity is defined as: ##EQU1##
The cut size (x.sub.50) is the particle size corresponding to .eta..sub.D =0.5 on the grade efficiency curve. Cut sharpness of the classification can be determined by intersecting the curve with the .eta..sub.D =0.25 and .eta..sub.D =0.75 lines and placing the particle sizes in the line intersections in relationship to each other. Cut sharpness (x.sub.25 /x.sub.75) is often used to quantify air classifier performance. x.sub.50 is the equiprobable cut size, i.e., the particle sizecorresponding to the 0.5 size selectivity value. x.sub.25 is the particle size corresponding to the 0.25 size selectivity value. x.sub.75 is the particle size corresponding to the 0.75 size selectivity value. Cut sharpness values range from 0.0 (almost no classification) to 1.0 (ideal but not achievable classification). In a production operation, an air classifier's cut sharpness typically ranges between 0.3 and 0.7. For a laboratory scale classifier, the cut sharpness can reach about 0.9. A good classifier has a wide adjustable cut size range and can achieve a very fine cut size and high cut sharpness.
There are a number of prior art references directed towards powder classifying apparatuses and methods. In general terms, most prior art powder classifiers comprise a means for dispersing the feed powder and a means for separating the dispersed powder at a specified cut size in order to obtain a coarse and a fines fraction. The prior art takes a variety of approaches in order to achieve the desired classification.
For example, a number of references disclose classifiers which employ the same basic design concept wherein a dispersion disk(s) is used to initially break up the feed powder and subsequently a classifying means such as a rotor is employed to impart a centrifugal force to the particles. The classification is typically achieved by applying a current of air to the dispersed powder stream, whereby the fine particles are removed from the particle stream by the air current and directed to a fines discharge outlet and the coarse particles travel through the air current and into a coarse particle discharge outlet. Among the references which describe variations of this basic design concept include U.S. Pat. Nos. 2,188,634; 2,542,095; 2,796,173; 3,720,313; 4,066,535; 4,100,061; 4,066,535; 4,388,183; 4,560,471; 4,604,192; 4,759,943; 4,869,786; and 5,024,754.
The references cited above provide a variety of designs in an attempt to optimize the same basic design concept. For example, some of the above references disclose designs wherein the current of air directs the fines inwardly towards the center of the classification chamber. see e.g. U.S. Pat. Nos. 4,560,471; 4,759,943; 2,796,173; 4,869,786 Others of the references disclose designs wherein the current of air directs the fines to an outer portion of the classifying chamber. see e.g. U.S. Pat. No. 4,066,535; 4,388,183. Many of the prior classifiers disclosed in the above references exploit the effects of gravity in that upon classification of the powder, the fines fraction and the coarse fraction are directed to separate discharge ports located in the bottom portion of the classifier housing. see e.g. U.S. Pat. Nos. 4,066,535; 4,388,183; 4,560,471; 4,759,943, 5,024,754. However, there are some prior art classifiers wherein the fine material is lifted upwardly against the force of gravity and is discharged from the upper portion of the classifier. see e.g. U.S. Pat. No. 4,661,244. A number of the references mentioned above disclose classifier systems wherein the dispersion means and the classifying means are separately driveable in order to achieve optimum particle dispersion and classification. see e.g. U.S. Pat. Nos. 5,024,754; 4,869,786; 4,661,244; 4,388,183; 4,100,061; 2,188,634.
However, there remains a need for an improved powder classifier which allows for control of a number of variables in order to obtain a more precise cut of the coarse fraction and fines fraction while also maintaining a high throughput of the feed powder. The present invention provides a novel design for such a classifier, the features of which are not disclosed or suggested by any of the prior art classifiers, either alone or in combination.